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Date: Wed, 31 Mar 2021 16:42:19 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Andrew Morton <akpm@...ux-foundation.org>, qianjun.kernel@...il.com
Cc: ast@...nel.org, daniel@...earbox.net, kafai@...com,
songliubraving@...com, yhs@...com, andriin@...com,
john.fastabend@...il.com, kpsingh@...omium.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
bpf@...r.kernel.org
Subject: Re: [PATCH V2 1/1] mm:improve the performance during fork
On 3/31/21 2:11 PM, Vlastimil Babka wrote:
> On 3/31/21 7:44 AM, Andrew Morton wrote:
>> On Mon, 29 Mar 2021 20:36:35 +0800 qianjun.kernel@...il.com wrote:
>>
>>> From: jun qian <qianjun.kernel@...il.com>
>>>
>>> In our project, Many business delays come from fork, so
>>> we started looking for the reason why fork is time-consuming.
>>> I used the ftrace with function_graph to trace the fork, found
>>> that the vm_normal_page will be called tens of thousands and
>>> the execution time of this vm_normal_page function is only a
>>> few nanoseconds. And the vm_normal_page is not a inline function.
>>> So I think if the function is inline style, it maybe reduce the
>>> call time overhead.
>>>
>>> I did the following experiment:
>>>
>>> use the bpftrace tool to trace the fork time :
>>>
>>> bpftrace -e 'kprobe:_do_fork/comm=="redis-server"/ {@...nsecs;} \
>>> kretprobe:_do_fork /comm=="redis-server"/{printf("the fork time \
>>> is %d us\n", (nsecs-@st)/1000)}'
>>>
>>> no inline vm_normal_page:
>>> result:
>>> the fork time is 40743 us
>>> the fork time is 41746 us
>>> the fork time is 41336 us
>>> the fork time is 42417 us
>>> the fork time is 40612 us
>>> the fork time is 40930 us
>>> the fork time is 41910 us
>>>
>>> inline vm_normal_page:
>>> result:
>>> the fork time is 39276 us
>>> the fork time is 38974 us
>>> the fork time is 39436 us
>>> the fork time is 38815 us
>>> the fork time is 39878 us
>>> the fork time is 39176 us
>>>
>>> In the same test environment, we can get 3% to 4% of
>>> performance improvement.
>>>
>>> note:the test data is from the 4.18.0-193.6.3.el8_2.v1.1.x86_64,
>>> because my product use this version kernel to test the redis
>>> server, If you need to compare the latest version of the kernel
>>> test data, you can refer to the version 1 Patch.
>>>
>>> We need to compare the changes in the size of vmlinux:
>>> inline non-inline diff
>>> vmlinux size 9709248 bytes 9709824 bytes -576 bytes
>>>
>>
>> I get very different results with gcc-7.2.0:
>>
>> q:/usr/src/25> size mm/memory.o
>> text data bss dec hex filename
>> 74898 3375 64 78337 13201 mm/memory.o-before
>> 75119 3363 64 78546 132d2 mm/memory.o-after
>
> I got this:
>
> ./scripts/bloat-o-meter memory.o.before mm/memory.o
> add/remove: 0/0 grow/shrink: 1/3 up/down: 285/-86 (199)
> Function old new delta
> copy_pte_range 2095 2380 +285
> vm_normal_page 168 163 -5
> do_anonymous_page 1039 1003 -36
> do_swap_page 1835 1790 -45
> Total: Before=42411, After=42610, chg +0.47%
>
>
>> That's a somewhat significant increase in code size, and larger code
>> size has a worsened cache footprint.
>>
>> Not that this is necessarily a bad thing for a function which is
>> tightly called many times in succession as is vm__normal_page()
>
> Hm but the inline only affects the users within mm/memory.c, unless the kernel
> is built with link time optimization (LTO), which is not AFAIK not the standard yet.
So I tried to inline the fast path of vm_normal_page() for every caller, see
below. The difference is only on architectures with CONFIG_ARCH_HAS_PTE_SPECIAL
where the fast path doesn't even need to look at vma flags. Of course inlining
has size costs, but there might be performance benefits, so you might want to
try measuring if it's worth it and I should make it a formal patch.
It might be also even better if we give up on the highest_memmap_pfn check or
make it CONFIG_DEBUG_VM only.
> ./scripts/bloat-o-meter vmlinux.before vmlinux.after
add/remove: 1/2 grow/shrink: 27/3 up/down: 2796/-479 (2317)
Function old new delta
collapse_pte_mapped_thp 1141 1364 +223
khugepaged_scan_pmd 1532 1738 +206
__vm_normal_page - 168 +168
pagemap_pmd_range 1679 1835 +156
__collapse_huge_page_isolate 1485 1628 +143
follow_page_pte 1454 1596 +142
queue_pages_pte_range 774 906 +132
clear_refs_pte_range 944 1061 +117
do_numa_page 643 758 +115
__munlock_pagevec_fill 438 551 +113
do_wp_page 567 676 +109
copy_pte_range 1953 2055 +102
madvise_free_pte_range 2053 2154 +101
gather_pte_stats 693 793 +100
zap_pte_range 1914 2000 +86
mc_handle_present_pte 144 230 +86
smaps_hugetlb_range 322 407 +85
clear_soft_dirty 273 357 +84
get_gate_page 668 751 +83
madvise_cold_or_pageout_pte_range 2885 2966 +81
get_mctgt_type 576 657 +81
change_pte_range 1300 1376 +76
smaps_pte_entry.isra 455 513 +58
wp_page_copy 1647 1696 +49
pud_offset.isra 126 167 +41
do_swap_page 1754 1790 +36
free_pud_range 1050 1065 +15
arch_local_irq_enable 136 144 +8
__pmd_alloc 670 668 -2
__handle_mm_fault 1754 1738 -16
mem_cgroup_move_charge_pte_range 1661 1535 -126
mc_handle_swap_pte.constprop 167 - -167
vm_normal_page 168 - -168
Total: Before=29768466, After=29770783, chg +0.01%
----8<----
>From f70bda6fbb7f17e13f3fa88fac203b7f426d0752 Mon Sep 17 00:00:00 2001
From: Vlastimil Babka <vbabka@...e.cz>
Date: Wed, 31 Mar 2021 15:59:15 +0200
Subject: [PATCH] inline vm_normal_page()
---
include/linux/mm.h | 18 +++++++++++++++++-
mm/internal.h | 2 --
mm/memory.c | 2 +-
3 files changed, 18 insertions(+), 4 deletions(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 3e4dc6678eb2..1df6ce4ab668 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1711,8 +1711,24 @@ struct zap_details {
pgoff_t last_index; /* Highest page->index to unmap */
};
-struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
+struct page *__vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
pte_t pte);
+extern unsigned long highest_memmap_pfn;
+static inline struct page *vm_normal_page(struct vm_area_struct *vma,
+ unsigned long addr, pte_t pte)
+{
+ unsigned long pfn;
+
+ if (IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL)
+ && likely(!pte_special(pte))) {
+ pfn = pte_pfn(pte);
+ if (likely(pfn <= highest_memmap_pfn))
+ return pfn_to_page(pfn);
+ }
+
+ return __vm_normal_page(vma, addr, pte);
+}
+
struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr,
pmd_t pmd);
diff --git a/mm/internal.h b/mm/internal.h
index 547a8d7f0cbb..cca1cbc3f6fa 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -117,8 +117,6 @@ static inline bool is_page_poisoned(struct page *page)
return false;
}
-extern unsigned long highest_memmap_pfn;
-
/*
* Maximum number of reclaim retries without progress before the OOM
* killer is consider the only way forward.
diff --git a/mm/memory.c b/mm/memory.c
index 5c3b29d3af66..d801914cfce4 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -603,7 +603,7 @@ static void print_bad_pte(struct vm_area_struct *vma, unsigned long addr,
* PFNMAP mappings in order to support COWable mappings.
*
*/
-struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
+struct page *__vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
pte_t pte)
{
unsigned long pfn = pte_pfn(pte);
--
2.30.2
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